For v1.1.0rfc3, series v0

This patch series (which is really the next-to-last two: [1f32f42](https://github.com/ajnelson/dfxml_schema/commit/1f32f4eff1215c94ff48b39076c0c18f74efeb53) and [63162cf](https://github.com/ajnelson/dfxml_schema/commit/63162cfd8825db22e5a06d600e3c05354762d6cf)) introduces some allowances for DFXML extensibility.  The schema's other-namespace parsing requirements are set to "lax" to allow for arbitrary extensibility, in some elements.  "Strict" makes introducing and validating new namespaces quite cumbersome.  (The validation essentially creates a lattice structure of XML namespaces, awkward to implement and record.)

Accepting these changes means the first official DFXML schema will allow for extensibility of the language.  **I plan to merge these as RFC-3 unless there is an objection by Sep. 27 in the afternoon Eastern time.**

I welcome discussion or acks on this patch series; note these commits will change after Sep. 20 in the afternoon Eastern time, when the [series](https://github.com/dfxml-working-group/dfxml_schema/pull/6) for RFC-2 is merged).